Document that the msun tests require WARNS=0

ATF tests have a default WARNS of 0, unlike other usermode programs. This
change is technically a noop, but it documents that the msun tests don't
work with any warnings enabled, at least not on all architectures.

Reviewed by: ngie
MFC after: 3 weeks
Sponsored by: Spectra Logic Corporation
Differential Revision: https://reviews.freebsd.org/D9933